The invention relates to a low-pressure mercury vapor discharge lamp comprising a discharge vessel which is sealed in a vacuum-tight manner and in which during operation of the lamp a discharge is present, while this discharge vessel contains a small quantity of an alloy which forms with mercury an amalgam. Such a lamp is known from U.S. Pat. No. 4,157,485 (PHN 8057).
In the known lamp, due to the presence of the amalgam in the discharge vessel, the mercury vapor pressure is stabilized during operation of the lamp at a value near 6.10.sup.-3 torr over a wide temperature interval. At such a value, the lamp has the highest efficiency of the conversion of electrical energy supplied to the lamp into ultraviolet radiation. If the energy supplied to the lamp strongly increases or if the lamp is operated at an area having a comparatively high ambient temperature (such as in given luminaires), the light output of the lamp hardly decreases. In the known lamp, the discharge vessel contains an alloy which forms with mercury an amalgam, such as an amalgam consisting of mercury, indium and bismuth.
Although a satisfactory vapor pressure stabilization is obtained in the discharge vessel with the amalgam mentioned in that patent over a wide temperature interval, it has been found that especially with lamps that can be comparatively heavily loaded and comprise a tubular discharge vessel of comparatively small diameter the stabilization value of the mercury vapor pressure during operation is too low to obtain an optimum efficiency and a highest possible light output. Moreover, it has been found that the mercury vapor pressure at a temperature in the discharge vessel at the area of the amalgam of 100.degree. to 120.degree. C. even falls below the first-mentioned value of 6.10.sup.-3 torr. This is disadvantageous especially with compact low-pressure discharge lamps in which the tubular discharge vessel is surrounded by a glass envelope and the temperature in the discharge vessel during operation lies just in this range.